Two kinds of light emitting diodes (LED) exist today. One is based on organic materials (OLED), while the other one is based on inorganic materials. Both kinds of LEDs essentially work on similar principles wherein positive and negative charge carriers are injected into a semiconducting material and light emission occurs when the charge carriers recombine in the light emission zone of the device stack. Both methods have their own advantages and disadvantages. OLEDs are attractive because such devices need not be produced on a crystalline substrate, the cost of producing such devices is low, the devices are power efficient operating at low voltages, the devices have flexible potential, and the organic material enable devices that emit light in a variety of attractive colors. However, OLEDs devices lack very high brightness and are limited in lifetime. Inorganic LEDs, on the other hand, are attractive because they enable extremely robust devices, which can exhibit very high efficiencies. In addition, the lifetime of inorganic LED devices is extremely long. The notable drawbacks of inorganic LEDs include limitations in fabrication technology and the high cost of fabrication.
Until now, the main problem with fabricating inorganic LED technology for display applications was the drive circuit. Inorganic LEDs are generally made with compound semiconductor material including group III-V or II-VI materials, for example, Gallium Nitride (GaN). Compound semiconductor materials require very high temperature for processing (>700 C). The drivers made using compound semiconductor technology exhibit significantly higher voltages compared to the standard silicon technology such as complimentary metal-oxide semiconductor (CMOS) with single crystal Su, TFTs using a-Si or poly-Si. Therefore, a display device using just GaN had not been feasible. There have been many attempts to combine the Si technology for driving the GaN LEDs to fabricate displays, however, they have been largely unsuccessful. As such, inorganic LEDs are still not a part of the display industry.
The display industry continues to rapidly change with research and developments improvements which reduce power consumption, improve image resolution, and decrease device thickness. While at the same time, the emersion of new trending markets including wearable electronics with flexible and bendable displays, has begun to reshape technology requirements at many levels including the backplane level.
Generally, pixels in a flat panel display are arranged in a matrix form, and generate light (luminescence) upon electrical activation from an array of thin-film-transistors, also known as TFT backplane. A TFT backplane is an important part of display applications as it functions as a series of switches to control the current flowing to each individual pixel. Until recently, there have been two primary types of TFT backplane technologies, one using TFTs with amorphous silicon (a-Si) active layer and the other using TFTs with polycrystalline silicon (poly-Si) active layer. Currently, backplane technologies rely on the presence of conventional silicon materials. The present invention teaches methods of integrating TFTs for display applications using nonconventional materials.
Therefore, there exists a need to provide a feasible and economically viable method of fabricating display devices integrating inorganic LEDs with metal oxide semiconductors (metal oxide TFTs), such as zinc oxide (ZnO) and indium gallium zinc oxide (IGZO).
It is therefore a primary object of the present invention to provide a new and improved method for monolithically integrating inorganic LEDs with metal oxide TFTs using nonconventional materials with high transparency in the visible spectrum and controllable carrier concentration.
It is another object of the present invention to provide a new and improved method for fabricating flexible and transparent devices at lower temperatures.
It is another object of the present invention to provide an active matrix display incorporating an oxide TFT directly with a III-V (e.g. GaN) LED array.
It is another object of the present invention to provide an active matrix display with an extremely high brightness.
It is another object of the present invention to provide an active matrix display having higher contrast.
It is another object of the present invention to provide an active matrix display having longer lifetime.